1. Field of the Invention
The present invention relates to a high-frequency circuit device, and particularly to a high-frequency circuit device suitable for use in a terminal device for mobile communications.
2. Description of the Related Art
FIG. 13 is a plan view of a conventional high output amplifier. FIG. 14 is a circuit diagram showing an equivalent circuit of the high output amplifier.
In FIG. 13, reference numeral 100 indicates a high output amplifier, reference numeral 102 indicates a semiconductor substrate, reference numerals 104 indicate transistors, reference numerals 106 indicate microstrip lines each having a xcex/4 electrical length, which are respectively impedance converter circuits, reference numeral 108 indicates a signal terminal, reference numeral 110 indicates a signal output terminal, and reference numerals 112 indicate via holes, respectively. A chip size of the high output amplifier 100 is normally a square of 1 mm to several tens of mm in most cases. A ground conductor is formed on the back or reverse side of the semiconductor substrate 102. The thickness of the semiconductor substrate ranges from about 30 xcexcm to about 150 xcexcm.
The transistors 104 used as basic elements of the high output amplifier 100 are commonly very low in input/output impedance (25 xcexa9 or less). In addition to it, a plurality of transistors are normally arranged in parallel to increase an output voltage. Therefore, the impedance is further lowered.
On the other hand, since the characteristic impedance of an external circuit is set to 50 xcexa9 as a standard, it is necessary to provide means for matching the impedances of the two to each other. One used for its purpose is a method of using a microstrip line having a xcex/4 electrical length as an impedance converter circuit.
When the input impedances of each single transistors 104 are supposed to be 10 xcexa9 as shown in FIG. 14, they are respectively converted to 50 xcexa9 by xcex/4 electrical-length microstrip lines adjacent to the transistors 104, which in turn are combined into one. Thus, a method is adopted in which since the resultant characteristic impedance results in a half 25 xcexa9, it is converted to 50 xcexa9 by a xcex/4 electrical-length microstrip line adjacent to the signal input terminal 108, whereby matching with the external circuit is made. This method is commonly used in a high output amplifier because a design error is small and capacitors and inductors are unnecessary.
However, the method using the xcex/4 electrical-length microstrip lines as the impedance converter circuits has the following problems.
Firstly, the configuration shown in FIG. 13 is easy to form such an unwanted or unnecessary oscillation loop as shown in FIG. 14 and produce oscillations. The isolation (backward transfer characteristic) of each transistor may be set larger than forward gain to make it hard to produce the unwanted oscillation loop. However, the isolation of the transistor is commonly deteriorated as the frequency increases. It is thus difficult to ensure it. Assuming that the operating frequency is 5 GHz, for example, the gain of a high output transistor at 5 GHz ranges from 15 dB to 25 dB, whereas the isolation thereof ranges from xe2x88x9220 dBxcx9cxe2x88x9230 dB. It cannot be said that the difference between the two is sufficient.
Further, in the configuration shown in FIG. 13, an input-side matching circuit and an output-side matching circuit respectively need three-by-three xcex/4 electrical-length microstrip lines.
Assuming that the operating frequency is 5 GHz and the shortening coefficient of a waveform on the semiconductor substrate 102 is 1/2.5, the xcex/4 electrical-length microstrip line needs a length of 6 mm. When six xcex/4 electrical-length microstrip lines are used, a chip size becomes a square of several tens of mm even at the minimum. This might cause an obstacle to a reduction in size and cost. When the microstrip line is used as for an information terminal for mobile communications in particular, a reduction in size and cost is a large item to be considered from the viewpoint of design in a portable or cellular telephone or the like in particular. There were often difficulties in satisfying its design requirements.
Incidentally, Japanese Patent Application Laid-Open No. Sho 64-1301 describes an MIC isolator but has not a description thereof used in a distribution/synthetic circuit.
The present invention has been made to solve the above-described problems. Therefore, an object of the present invention is to obtain a high-frequency circuit device hard to produce unnecessary oscillations.
According to one aspect of the invention, there is provided a high-frequency circuit device comprising: a distribution circuit for distributing a signal inputted from a signal input terminal to a plurality of first lines through a branch portion; a synthetic circuit for combining signals inputted from a plurality of second lines into one through a combined portion as an output signal and outputting it from a signal output terminal; transistors respectively placed between one ends of each individual first lines of the distribution circuit and one ends of each individual second lines of the synthetic circuit; and isolators respectively connected between the transistors and the signal input terminal and between the transistors and the signal output terminal.
Accordingly, this inventive high-frequency circuit device makes it hard to form an unwanted oscillation loop. In its turn, a high-reliability high-frequency circuit device can be configured.
Another object of the present invention is to obtain a small-sized and low-cost high-frequency circuit device.
According to another aspect of the invention, there is provided a high-frequency circuit device comprising: a distribution circuit for distributing a signal inputted from a signal input terminal to a plurality of first lines respectively having a plurality of first impedance converter circuits through a branch portion; a synthetic circuit for combining signals inputted from a plurality of second lines each having a second impedance converter circuit into one through a combined portion as an output signal and outputting it from a signal output terminal; transistors respectively placed between one ends of each individual first lines of the distribution circuit and one ends of each individual second lines of the synthetic circuit; and isolators connected either between the transistors and the branch portion of the distribution circuit or between the transistors and the combined portion of the synthetic circuit.
Accordingly, this inventive high-frequency circuit device makes it hard to form an unwanted oscillation loop and allows a reduction in the number of the isolators, whereby a required substrate area can be reduced. In its turn, a high-reliability, and small and inexpensive high-frequency circuit device can be configured.
Other objects and advantages of the invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific embodiments are given by way of illustration only since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.